Decembee 2, 1898.] 



SCIENCE. 



773 



mosphere of the sun, by M. Janssen, the 

 well-known French astronomer ; and its 

 name was suggested by Messrs. Frankland 

 and Lockyer, in 1868, to characterize the 

 brilliant yellow line by which its presence 

 in the sun is revealed. Neither of these 

 elements has been combined with others, 

 although it is possible that each exists in 

 combination with one or more of the ele- 

 ments contained in the minerals from 

 which helium can be obtained by heating, 

 for it has been found that small quantities 

 ■of argon, along with considerable quantities 

 of helium, are evolved from such minerals. 

 Again, both of these elements possess one 

 curious property, which they share with 

 gaseous mercury alone, so far as is known, 

 among all elements. That is technically 

 called the ratio between their specific heats 

 at constant pressure and at constant volume. 

 It would be difficult here to set forth the 

 reasoning by which it is deduced that, inas- 

 much as the ratio for these gases is If to 1 

 between specific heat at constant pressure 

 and at constant volume, the molecules of 

 these elements, unlike those of oxygen and 

 hydrogen and the other commoner gases, 

 but like those of mercury gas, consist not of 

 agglomerations of two or more atoms, but 

 of single atoms. These characteristics at 

 once establish a connection between the 

 ■two elements helium and argon, and differ- 

 entiate them in kind from all other gaseous 

 elements. 



Now, taking the density of hydrogen as 

 unity, that of helium is very nearly 2 and 

 that of argon 20. And one of the conclu- 

 sions which follows from the Kinetic Theory 

 of Gases is that equal volumes of gases con- 

 tain equal numbers of molecules. Thus the 

 fact that helium is twice as heavy as 

 hydrogen carries with it the conclusion 

 that a molecule of helium is twice as heavy 

 as a molecule of hydrogen, whatever the 

 absolute weight of the latter may be. 



Now, it can be demonstrated that there 



is a strong probability in favor of the as- 

 sumption that a molecule of hydrogen con- 

 sists of two atoms, inseparable from each 

 other unless by combination with some 

 other element. And if a molecule of helium 

 consisting of one atom is twice as heavy as 

 a molecule of hydrogen consisting of two, 

 then it follows that an atom of helium is 

 four times as heavy as an atom of hydrogen; 

 in other words, the atomic weight of helium 

 is 4, that of hydrogen being taken as 1. 

 Similar reasoning proves the atomic weight 

 of argon to be 40, from the known fact that 

 it is twenty times as heavy as hydrogen. 

 Moreover, it is noteworthy that the differ- 

 ence between these numbers 40 and 4 is 36. 

 Mr. John Newlands, whose recent death 

 is deplored by the scientific world, as long 

 ago as 1863 brought forward what he 

 termed a ' law of octaves.' It consisted in 

 arranging the numbers which represent the 

 atomic weights of the elements in seven 

 rows, beginning again with the eighth ele- 

 ment, so that its atomic weight occupies a 

 position in the table below that of the first, 

 the ninth below the second, the fifteenth 

 again below the first, and so on. The re- 

 production of three of such rows will make 

 the meaning clear. 



Li 7 Be 9.2 B 11 C 12 N 14 O 16 F 19 



Na 23 Mg 24.3 Al 27 Si 28 P 31 S 32 CI 35.5 



K 39 Ca 40 So 44 Ti 46 V 52 Cr 52.5 Mn 55 



Etc. Etc. 



The elements appear in this table in 

 groups, of which the individual members 

 closely resemble each other, often in ap- 

 pearance, and always in the nature of the 

 compounds they form with other elements. 

 Thus, to take the first column, the three 

 elements lithium, sodium and potassium, 

 together with others not here produced, but 

 which occur later on in the table, rubidium 

 and CEesium, are all white waxy metallic 

 solids, easily cut with a knife, tarnishing 

 rapidly in contact with ordinary moist air, 

 and forming compounds which themselves 



